The major goal of this application is to understand the interplay between receptors and channels that control excitability of the thin fiber muscle afferents that evoke the exercise pressor reflex (EPR) as well as transduce the sensation of pain. The EPR, which arises from contraction of skeletal muscle, is one of two key neural mechanisms that evoke the cardiovascular adjustments to exercise. These adjustments, which support the ability of skeletal muscles to contract by increasing blood flow and oxygen to exercising muscles, include increases in systemic blood pressure, cardiac output and ventilation. With respect to the thin fiber muscle afferents that mediate the EPR, we combine the power of the in vitro whole-cell patch-clamp technique to determine the mechanisms of afferent excitability, with the physiological insights provided by in vivo electrophysiology to determine how these mechanisms translate into increased excitability. For in vitro experiments, muscle afferents will be identified by retrograde labeling DRG neurons with DiI that has been injected into the triceps surae muscles. Particular attention will be paid to small afferent neurons that express channels known to elicit the EPR (e.g. ASIC, TRPV1, P2X and TRPA1). For in vivo experiments, thin fiber triceps surae muscle afferents will be identified by their conduction velocities and their receptive fields. In both in vitro and in vivo experiments, we will concentrat on the mechanisms by which two receptors, the opioid (Aim 1) and ?-opioid (Aim 2) receptors, inhibit the EPR. Additional experiments will investigate the role of two voltage depend ion channels, KV7 (Aim 3) and NaV1.7 (Aim 4) in controlling the excitability of the EPR. The proposed experiments are natural extensions from those of our previous funding period and will greatly increase our understanding of the mechanisms affecting the excitatory afferents comprising the sensory arm of the EPR.